Klebsiella pneumoniae is a leading cause of community-acquired and nosocomial infections. Moreover, K pneumoniae is the second leading cause of nosocomial gram negative bacteremia. The recent emergence of multi-antibiotic resistant strains of K pneumoniae due to extended-spectrum B-lactamase (ESBL) production is cause for significant clinical concern. Interestingly, antibiotic resistance appears to be more prevalent in blood isolates than from other sources. While the innate host response towards gram-negative bacterial infections has been characterized, little is known about gamma delta-T cells and their role in these infections. Our preliminary studies indicate that gamma delta-T cell knockout (KO) mice have significantly impaired early expression of pulmonary and hepatic IFN-gamma and TNF-alpha mRNA following intratracheal K. pneumoniae infection, increased blood bacterial dissemination, and increased hepatic bacterial burden subsequent to the initial pulmonary infection. Additional studies indicate increased mortality following intravenous bacterial inoculation in gamma delta-T cell KO mice and uncontrolled blood bacterial growth. Combined, our preliminary data suggest gamma delta-T cell KO mice succumb from an impaired ability to clear disseminated bacteria rather than from an inability to clear the organism from the primary pulmonary infection. The hypothesis of this proposal is that gamma delta T cells play a critical role in the host acute inflammatory response during gram-negative bacteremia via recognition of heat shock protein 60 expression in the liver following infection. A murine model of blood-borne K pneumoniae infection will be used to perform the following Specific Aims: 1)To contrast the host response in gamma delta T cell knockout and wildtype mice during K pneumoniae bacteremia, 2) To assess the kinetics of gamma delta T cell A) recruitment and activation and B) cytokine production during K pneumonia bacteremia, 3) To reconstitute resistance to K pneumoniae in gamma delta T cell knockout mice by adoptive transfer of gamma delta T cells from wildtype and cytokine deficient (IFN-gamma or TNF-alpha) mice, 4) To confer resistance to K. pneumoniae in gamma delta T cell knockout mice by TNF-alpha or IFN-gamma reconstitution using systemic adenovirus gene therapy, and 5) To assess the requirement of heat shock protein 60 for gamma delta T cell activation during K. pneumoniae bacteremia. These studies will provide insights for the development of therapeutic modalities aimed at augmenting host responses, resulting in enhanced resolution of bacterial infections.